net: mana: Record the physical address for doorbell page region

For supporting RDMA device with multiple user contexts with their
individual doorbell pages, record the start address of doorbell page
region for use by the RDMA driver to allocate user context doorbell IDs.

Reviewed-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Long Li <longli@microsoft.com>
Link: https://lore.kernel.org/r/1667502990-2559-3-git-send-email-longli@linuxonhyperv.com
Acked-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>